This invention relates to a method of reducing the echoes from reflectors disposed outside the focal line in an echo-sounding process using an ultrasonic field focusing in a focal line. This invention also relates to apparatus for performing the method.
For the purpose of the present description, the term "cross-sectional area of an ultrasonic field" is defined as the surface of intersection between the ultrasonic field and a plane at right angles to the direction of propagation of the ultrasonic field.
Echo sounding processes using a convergent ultrasonic field having a substantially annular cross-sectional area are efficient in focusing the ultrasonic field through a great depth. It has been found, however, that if the ultrasonic wave used is in phase over the entire cross-sectional area and has constant amplitude, relatively large lateral maxima may occur, i.e. undesired echoes from reflectors at the side of the focal line may have a relatively large amplitude.
To reduce the echoes from reflectors outside the focal line it has already been proposed to transmit, after transmitting a first pulsed, limited ultrasonic wave which is in phase over the entire cross-sectional area, at least a second pulsed and limited ultrasonic wave which is not in phase over the entire cross-sectional area; the electric signals obtained from the echoes are provided with weighting coefficients and echoes having the same travel time are added (see e.g. Swiss Pat. No. 549,220 and U.S. Pat No. 3,836,948).
In another known method, at least two pulsed and limited ultrasonic waves are transmitted in succession, the wave amplitudes being functions of the angle .phi. measured in the cross-sectional area, and echoes from reflectors in and outside the focal line are received and converted into electric echo signals which are multiplied by a function of the angle .phi., after which the echoes having the same travel time are added (see German Offenlegungsschrift No. 2,351,352 and British patent specification No. 1,413,740).
These two known methods can greatly or almost completely eliminate echoes from reflectors outside the focal line. However, apparatus for performing these known methods requires relatively expensive circuitry, which is not always justified in view of the circumstances in which the apparatus is used.